1. Field of the Invention
The present invention relates to a containment and pressure suppression arrangement for a consolidated nuclear steam system.
2. Summary of the Prior Art
A consolidated nuclear steam system (CNSS) incorporates a nuclear core, steam generator and reactor coolant pumps within a single pressure vessel, thereby achieving a compact arrangement and minimizing the use of auxiliary equipment. The CNSS is particularly beneficial in marine propulsion applications where reduced size and weight decrease the capital cost of the power plant and the ship, and increase the available space for cargo and passenger accommodations.
A compact pressure suppression system, similar to those used for stationary nuclear power plants, provides a secondary containment for the CNSS.
The main purpose of the CNSS containment is to confine and control potential releases of radioactivity from the reactor coolant system following a postulated occurence known in the art as a loss of coolant accident (LOCA).
The containment often includes two compartments commonly designated as the dry well and the wet well, the latter so called since it contains a liquid pressure suppression pool. All flow process penetrations into the pressure vessel are made within the dry well compartment.
Thus, the dry well receives the discharge of the reactor coolant in the event a loss of coolant accident (LOCA). Reactor coolant effluent, in the form of steam and water, is discharged into the dry well during a LOCA, mixes with the air therein, and pressurizes this compartment. A set of vent pipes generally directs the effluent from the dry well into the liquid pressure suppression pool of the wet well. The vent pipe discharges are submerged in the wet well water in order to condense the effluent, thereby serving to limit peak pressure and temperature. The water in the pressure suppression pool also functions as a radiation shield.
A number of variations in pressure suppression and containment systems embodying the principles described above are known in the prior art. In some arrangements, the containment shell is a free standing steel cylinder supported at its bottom with a main operating floor approximately half-way up the containment dividing it into two compartments. An annular wet well is formed by the cooperation of a second cylindrical shell that is disposed below the operating floor, located as close to the reactor as feasible, with the containment wall or outer cylindrical shell. The lower portion of the wet well is filled with water and constitutes the vapor suppression pool. The annular wet well is sub-divided into separate chambers by vertical baffle plates which extend upwardly from the bottom of the containment and one vent pipe discharges into each chamber. The chambers are arranged so that the discharge point of the vent pipe remains submerged during ship motion. However, an air space is generally provided above the suppression pool within the wet well in order to allow for the volumetric compression of the air forced into the wet well from the dry well with the reactor effluent during a LOCA. During normal operations, therefore, suppression pool water will shift as a result of the pitch and roll of the ship. Low pressure rupture discs in the vent pipes normally separate the dry and wet well in order to prevent the suppression pool water from flowing into the dry well at extreme ship attitudes.
A major objective in CNSS containment designs for ship propulsion systems is to afford maximum protection from the ionizing radiation emanating from the reactor core with minimum radiation shield weight. The primary shielding includes all the materials of the containment vessel and, as noted above, the annular shield provided by the water of the vapor suppression pool. Hence, shifting of part of the water in the wet well in conjunction with the pitch and roll of the vessel results in a discontinuity of the shielding about the core. In the prior art, the reactor containment has been surrounded by a heavy concrete biological shield designed to limit radiation dose rates in the surrounding ship areas.